Long-range ultrasonic testing (LRUT) is one of nondestructive testing methods for cost-effective detection of defects, such as corrosions, cracks, and the like, using low frequency (typically 100 kHz or less) guided waves propagating far along the length of the structure, such as a pipeline, a pipe, a rope, a plate, a shell, or the like.
Guided waves, such as guide waves of fundamental torsional mode (T(0,1) mode) and fundamental shear horizontal mode (SH0 mode), have proven to be particularly useful for LRUT of cylindrical structures and plate structures. This is because such guided waves have a desirable characteristic that their propagation speed is constant irrespective of frequency and they have a relatively uniform particle displacement in a section of the structure perpendicular to the propagation direction and are insensitive to fluid which may be in/out of the structure.
One of the most important characteristics required for an effective LRUT system would be the ability to transmit a single-mode guided wave pulse in an intended direction and identify and detect the same-mode echoes which have propagated in an intended direction.
A phased array probe, which is widely used to obtain such transmission and reception directivity, is composed of the same transducers of two channels acoustically coupled to a surface of the structure and operated in a pulse-echo mode, and the spacing between the two channels corresponds to a quarter wavelength at the center frequency of the guided wave pulse.
A prior art for operating such a phased array probe induces constructive interference between guided waves of two channels propagating in the intended direction (forward direction) and destructive interface between guided waves propagating in a direction (backward direction) opposite to the intended direction, thereby allowing an LRUT system to have transmission directivity. In addition, in the receiving process, a channel that first detects forward echoes is selected, phases of signals of the selected channel are delayed by 90 degrees, and then the phase-delayed channel signals and the other channel signals are summed up, thereby enhancing forward echo signals and suppressing backward echo signals.
Meanwhile, a magnetostrictive transducer used in the LRUT system is composed of an object having excellent magnetostrictive properties, a permanent magnet (or electromagnet) as a means of applying a bias static magnetic field to the magnetostrictive object, and a radiofrequency (RF) coil which applies a dynamic magnetic field to a region of within the bias magnetic field or detects a change in magnetic field caused by a guided wave entering the region of the bias magnetic field.
In the case of a structure made of a ferromagnetic material, such as iron, a noncontact transducer may be realized by using the structure itself as an element of the transducer. In the case of a structure made of a nonmagnetic material, thin (usually 0.2 mm or less) strips (or patches) of nickel or iron-cobalt alloys adhered to a surface of the structure may be used. Such a magnetostrictive patch transducer has been widely used for LRUT of steel structures. This is because these magnetostrictive materials have superior magnetostrictive properties as compared with iron and allow easy formation of magnetic fields. Also, according to the relative directions of the static magnetic field and the dynamic magnetic field, the magnetostrictive patch transducer was able to selectively transmit and receive various guided wave modes. Due to the Wiedemann effect, the above transducer, driven by two mutually perpendicular magnetic fields, was particularly suitable for transmitting and receiving the SH0 and T (0,1) modes.
A phased array magnetostrictive probe composed of the same two magnetostrictive strip transducers have been widely used in LRUT of pipelines. The individual transducer is composed of an iron-cobalt alloy strip which is bonded to an outer surface of the pipeline so as to surround the circumference thereof, and a solenoid-type RF coil which surrounds the bonded strip and forms a dynamic magnetic field on the strip in the width direction of the strip (a pipeline longitudinal direction). Residual magnetization in the strip is used as the bias magnetic field directed to the strip longitudinal direction (a pipe line circumferential direction) required for operation of a guided wave of T (0,1) mode. This residual magnetization is formed by moving a U-shaped permanent magnet along the two bonded strips at a uniform speed once or twice.
Recently, a phased array magnetostrictive probe has also been used which consist of a magnetostrictive band that include one or more solenoids closely surrounding a magnetostrictive strip or patch, and elongated spiral surface RF coils of two channels. A bias magnetic field may be optimized by flowing a direct current through the solenoids and the spacing between two legs of the spiral coil which corresponds to a half wavelength at the center frequency of the guided wave pulse results in an improved signal-to-noise ratio.
Main operating parameters, such as the spacing for transducers, a driving time difference for electric pulses for driving the transducers, and a phase delay for echo signals are all determined by taking into consideration the center frequency characteristic of the intended guided wave pulse. This means that the frequency characteristics of the transducer are very important in improving the transmission and reception directivity for efficient long-range ultrasonic testing. The reflectivity of a guided wave of T (0, 1) mode due to the thickness variation in a pipe specimen having a stepwise thickness variation depends on the incident direction and frequency thereof. That is, the reflectivity for the wave incident from a thick region increases with increasing frequency while the reflectivity for the wave incident from a thin region rather decreases with increasing frequency. The former is a universal property of wave reflection that can be predicted, and the latter is due to the geometric acoustical properties of the guided wave. Similar reflection characteristics will appear at both ends of the magnetostrictive patch bonded to the structure surface, which will have a considerable impact on the frequency characteristics of the phased array magnetostrictive probe. This effect may increase with the increasing wave frequency or the thickness ratio of the magnetostrictive patch to the structure.